Which exercise is better: with heavy weights and less reps, or with lighter weight and more reps? How will either of these two approaches affect my training outcomes?
There are two factors that will be important to contributing an answer to your question: your genetic predisposition and your goal.
In either case, the intensity with which you train is a determining factor. How you manage this intensity, whether you will respond better to progressively increasing the amount of weight that you train with or whether you will make better progress by manipulating training volume does in fact rest on genetic factors.
Gene tested exercise prescription is still in its relative infancy, still more of an art than a science and it may be a while before you can spit in a sample bottle and then go down to your nearest genetic exercise councillor to get your exercise and diet prescription satisfactorily sorted out.
But current genetic testing technologies and data bases can often offer valuable clues. Some trainers have the experience, talent and knack to glean good educated guesses and perhaps be helpful in constructing useful protocols from the currently available tech.
For example, the MSTN gene, which famously regulates the production of Myostatin, a biogenic compound responsible for putting the brakes on muscle growth, will contribute to hypertrophic response, particularly if you lack it.
In lieu of genetic testing and interpretation, your own training experience will be instructive in determining what type of training is likely to result in the improvement and gains that you are after. As mentioned, also important to how you train is what your primary goal in training is and what you are aiming at achieving. Do you want to emphasise hypertrophy, strength, power, endurance or a combination of these factors? Your answers here will be helpful in putting together a realistic training strategy.
If all this sounds like a waffle and a way to avoid a simple and direct answer to what you thought was just a simple question, you are perhaps partially right. But rather than give you a simplistic answer that might or might not score a direct hit, or one that might waste many valuable hours of your time in futile effort, my object here is to make the point: “how you should train depends…”